Although blockchains are highly secure, so is a computer with no electricity. You may find both proponents of blockchain discussing its immutability, defending the consensus protocols and successful decentralization, and opponents citing the numerous hacks. In short, blockchain is no more secure than its users. Due to its decentralized nature, however; no single entity can compromise an entire network, while a single engineer at LinkedIn may accidentally have logged and leaked millions of passwords, in blockchain only the holder of a private key is responsible for a wallet.